Many or most of the short-span bridge structures in the United States are constructed of a deck surface on top of a supporting structure, most commonly a framework of steel or prestressed concrete I-beams. For example, a conventional two-span bridge (a total span of 140 feet) could have a 3" pavement wearing surface on a 7" structural slab of reinforced concrete supported on top of a framing system consisting of five longitudinal 36" steel wide flange beams or five longitudinal 45" type IV AASHTO prestressed concrete girders.
There is believed to be a significant need in the U.S. for a structural beam for use in the framework of a bridge that provides greater resistance to corrosion through the use of plastic, and that can be built not only at a competitive cost, but also with a reduction in the self weight of the structural members as it relates to transportation and erection costs. Of course plastic can also refer to fiber reinforced plastic.
It has been known that fabrication of structural elements from fiber reinforced plastics results in a structure that is less susceptible to deterioration stemming from exposure to corrosive environments. One type of structural framing member is currently manufactured using the pultrusion process. In this process, unidirectional fibers (typically glass) are pulled continuously through a metal die where they are encompassed by a multidirectional glass fabric and fused together with a thermosetting resin matrix such as vinyl ester. Although the composite structural members offer enhanced corrosion resistance, it is well known that structural shapes utilizing glass fibers have a very low elastic modulus compared to steel and very high material costs relative to both concrete and steel. As a result, pultruded structural beams consisting entirely of fiber reinforced plastic cannot be cost effectively designed and fabricated to meet the serviceability requirements, i.e. live load deflection criteria, currently mandated in the design codes for buildings and bridges.